Polyolefin polymers are well known and are useful in many applications. In particular, linear low-density polyethylene polymers (LLDPE) are in demand because they possess properties that distinguish them from other polyethylene polymers, such as branched ethylene homopolyethylene polymers (low density polyethylene, LDPE). LLDPE has a density of 0.910 to 0.945 g/cm3.
The market for LLDPE has grown rapidly, particularly for applications such as blown and cast films, injection molding, rotational molding, blow molding, pipe, tubing, and wire and cable applications. A principal area for LLDPE copolymers is in film forming applications because these co-polymers typically exhibit high dart impact, high Elmendorf tear, high tensile strength and high elongation, in both the machine direction (MD) and the transverse direction (TD), compared with counterpart LDPE resins. U.S. Pat. No. 4,076,698, by Anderson et al., describes some of these properties. Increasingly, there is a demand in the market for high performance LLDPE resin having better impact strength, higher transparency, and less wax content.
Much of the effort to improve LLDPE properties has been directed at developing new catalyst systems for producing ethylene co-polymer having a narrow molecular weight distribution and/or a narrow branching compositional distribution. Single site catalyst systems employing organometallic compounds such as metallocenes or methylaluminoxane (MAO) have been shown to provide superior control of these parameters. However, single site catalyst systems have not been widely employed in gas phase processes, which are the most widely used industrial means of producing commodity LLDPE. Commercial use of single site catalyst systems has been hindered because these systems are difficult to incorporate into existing plants. Furthermore, many organometallic compounds are difficult to immobilize onto supports because they are insoluble in aliphatic hydrocarbon solvents.
Titanium based Ziegler-Natta type catalyst systems are well known in the art and have the advantage of being applicable in existing gas phase processes. An example of such a catalyst system is described in U.S. Pat. No. 3,113,115, by Ziegler et al. Much effort has been directed at developing Ziegler-Natta catalyst systems to produce ethylene (co)-polymer having narrow molecular weight and/or branching compositional distributions.
U.S. Pat. Nos. 5,260,245, 5,336,652, and 5,561,091, by Mink et al., describe a catalyst system in which dialkylmagnesium and silane compounds are reacted with the OH groups of a silica support, which is then contacted with a transition metal halide to form a relatively homogeneous active site. This catalyst system produces more homogeneous ethylene polymer or co-polymer than does the traditional magnesium-titanium(IV) halide based Ziegler-Natta system.
U.S. Pat. No. 5,047,468, by Lee et al., describes a catalyst system for producing LLDPE. The catalyst is obtained by dissolving MgCl2 with [TiCl3(AlCl3)1/3] in THF to make a solution containing MgCl2 and titanium halide, which is then immobilized on silica.
U.S. Pat. Nos. 5,091,353 and 5,192,731, by Kioka et al., describe a catalyst system in which a magnesium compound is contacted with an organnoaluminum compound to produce a solid magnesium aluminum complex, which is then reacted with tetravalent titanium to afford a solid catalyst.
U.S. Pat. No. 4,748,221 and European Patent No. 0 703 246 A1 describe a catalyst system in which magnesium metal is reacted with butylchloride in a non-polar solvent. The reaction is initiated with Ti(OR)4 and sustained by further treatment with TiCl4/Ti(OR)4/BuCl to produce a catalyst suitable for ethylene co-polymerization in a gas phase process.
The references described above are directed at improving titanium catalysts by controlling the solid catalyst formation process in an attempt to achieve a more homogeneous active site, leading to greater control of the molecular weight distribution and/or branching compositional distribution of the resulting polymers. Such control is difficult with supported catalysts because active site formation is greatly influenced by the interaction of the titanium complex with the heterogeneous surface of the magnesium halide support. This effect is so great that the process of immobilizing the titanium component onto the magnesium halide support often overwhelms the original coordination properties of titanium component. Attempts to control the coordination environment of the titanium component often fails to achieve improved catalyst properties because the effects of these improvements are lost during the immobilization process. Meticulous control of the catalyst preparation process has been required to ensure homogeneous active site formation during immobilization in titanium-magnesium halide based Ziegler-Natta catalyst.
U.S. Pat. Nos. 6,500,906 B1 and 6,590,046 B2, by Kong et al., describe a catalyst system that does not require immobilization of the titanium component onto the magnesium halide support prior to use. The catalyst system employs a liquid phase transition metal solution in-situ with solid magnesium halide support. The transition metal solution is prepared by reacting Mg[AlR′(OR)3]2 with a nitrogen bound chelating ligand (N-chelate ligand) such as dialkylcarbodiimide, and then with MX4 as described in equation (1), where M is Ti or Zr. This method has the advantage of not relying on a complicated immobilization process to ensure improved catalyst properties.
There is a need in the art for titanium base catalyst systems capable of providing polymers having controlled molecular weight distributions and/or compositional distributions. Such catalysts will require that the coordination properties of the titanium complexes not be deteriorated upon immobilizing the titanium complex on a magnesium halide support. Ideally, methods of providing such catalysts will not depend on meticulous control of the catalyst formation process, as such control is difficult and expensive to exert in the context of large-scale commercial utilization.